This invention relates to a fossil-fired thermal system such as a power plant or steam generator consisting of one or more heat exchangers each constructed of heat exchanger tubes which confine the systems"" working fluid, fluid being heated by the products of combustion, and, more particularly, to a method for rapid detection of tube failures when they occur and the location of the heat exchanger having the tube failure, without need for direct instrumentation, thereby preventing more serious damage and minimizing repair time on the effected heat exchanger.
Although especially applicable to xe2x80x9cThe Input/Loss Methodxe2x80x9d as installed at coal-fired power plants, this invention may be applied to any on-line monitoring method and any other of the xe2x80x9cInput/Loss methodsxe2x80x9d, installed at any thermal system burning a fossil fuel. Such monitoring is assumed to be conducted in a continuous manner (i.e., on-line), processing one monitoring cycle after another, each cycle includes determining stoichiometric balances of the combustion process and, specifically, the fuel""s heating value. Specifically, The Input/Loss Method and its associated technologies are described in the following U.S. patent applications Ser. No. 10/131,932 (hereinafter termed ""932), Ser. No. 09/273,711 (hereinafter termed ""711), Ser. No. 09/630,853 (hereinafter termed ""853), and Ser. No. 10/087,879 (hereinafter termed ""879); and in their related provisional patent applications and Continuation-In-Parts. One of the Input/Loss methods, a rudimentary method, is described in U.S. Pat. No. 5,367,470 issued Nov. 22, 1994 (hereinafter termed ""470), and in U.S. Pat. No. 5,790,420 issued Aug. 4, 1998 (hereinafter termed ""420).
Large fossil-fired thermal systems, and especially coal-fired power plants, having large heat exchangers, are prone to tube leaks of their working fluid (typically water as liquid or steam). These tube leaks represent a potential for serious physical damage to heat exchangers due to pipe whip (i.e., mechanical movement) and/or steam cutting of metal given high leakages flowing at critical velocities. In a modern steam generating system, pressures of the working fluid commonly exceed 2300 psia; for a super-critical system, pressures of 3300 psia are not uncommon. Given failure of a heat exchanger tube, such fluid will experience many times critical pressure ratio as it expands into the combustion gases; that is, mixing with the products of combustion at essentially atmospheric pressure. When undetected, the damage from such tube failures may range from $2 to $10 million/leak forcing the system down for major repairs lasting up to a week. If detected early, tube failures may be repaired before catastrophic damage, such repairs lasting only several days and costing a fraction of the cost associated with late detection and catastrophic damage. Repair times may be further reduced if the location of the heat exchanger which has the leak is identified before repairs are initiated.
Tube failures in large steam generators are typically caused by one the following general categories:
Metallurgical damage caused by hydrogen absorption in the metal resulting in either embrittlement or the formation of non-protective magnetite;
Caustic gouging caused by the presence of free hydroxide in the water;
Corrosion-fatigue damage caused from the water-side of the tube, compounded by stress;
Corrosion damage caused by impacts from solid ash particles;
Fatigue failure caused by oxidation and/or mechanical movement, compounded by stress;
Overheating (e.g., from tube blockage) causing local creep; and
Physical damage from steam cutting and/or mechanical movement associated with another failed tube in the same local.
Commonly, the physical leak initiates as a relatively small penetration, although initial breaks may also occur. For reference and further discussion see: Chapter 18, xe2x80x9cFailure Analysis and In-Service Experiencexe2x80x94Fossil Boilers and Other Heat Transfer Surfacesxe2x80x9d of The ASME Handbook on Water Technology for Thermal Power Systems, P. Cohen, Editor, The American Society of Mechanical Engineers, New York, N.Y., 1989.
Present industrial art has practiced the detection of tube failures by either: acoustic monitoring devices; through water balance testing; or through the monitoring of effluent water using instrumentation mounted in the xe2x80x9csmoke Stackxe2x80x9d. Acoustic devices detect the unique noise created by fluids at high velocities. However, acoustic devices rarely work in large steam generators, are expensive and require benchmarking with known acoustical signatures. Water balance testing may be conducted periodically on the entire system through which large water losses due to tube failures might be discovered. However, water balance testing is expensive, insensitive to small leaks, and typically may not be conducted at sufficient frequency to prevent serious damage. The use of an effluent water instrument has been shown to be sensitive to tube failures. However, effluent water instrumentation may not differentiate between originating sources of water (e.g., between high humidity in the combustion air, or high fuel water, or a tube leakage). In practice all known methods suffer serious short-comings and are not reliable in detecting early tube failures.
The patents ""470 and ""420 make no mention of heat exchanger tube failures nor their detection. Although the technologies of Applications ""711 and ""853 support this invention, they make no mention of tube failures, their detection nor their location. Applications ""932 and ""879 support this invention directly. Although the methods of ""932 and ""879 are useful, the present invention further improves these methods. There is no established art related to this invention; there is clear need for early detection of tube failures and their location within fossil-fired systems.
This invention relates to a fossil-fired thermal system such as a power plant or steam generator, and, more particularly, to a method for rapid detection of tube failures and their location, without direct instrumentation, thereby preventing serious damage and minimizing repair time on the effected heat exchanger. Tube failures are detected through use of combustion stoichiometrics, in combination with an ability to correct effluent data through use of optimization procedures. The location of the failure within a steam generator is determined through use of energy balances, and iterative techniques. Further, this invention teaches how the stoichiometric mechanism of tube failure may be identified and reported to the system operator. This invention addresses the deficiencies found in all present detection methods.
Effluent water concentration (at the Stack) may consist of any one or all of the following sources of working fluid (assuming the working fluid is water): heat exchanger tube leaks; water added at the point of combustion (e.g., steam used to atomize fuel); pollutant control processes resulting in the in-flow of water; and/or soot blowing processes using water to clean heat exchanger surfaces (typically used in coal-fired systems). These sources of working fluid are in addition to: water formed from the combustion of hydrocarbon fuels; free water born by the fuel; and moisture carried by combustion air including air leakage. All such sources of effluent water are addressed by this invention through combustion stoichiometrics in combination with an ability to correct effluent data through use of optimization procedures.
This invention adds to the technology associated with Input/Loss methods. Specifically The Input/Loss Method has been applied through computer software, installable on a personal computer termed a xe2x80x9cCalculational Enginexe2x80x9d, and has been demonstrated as being highly useful to power plant engineers. The Calculational Engine receives data from the system""s data acquisition devices. The Calculational Engine""s software consists of the EX-FOSS, FUEL and HEATRATE programs described in ""711, and in FIG. 2 herein, and the ERR-CALC program described in ""932 and ""879, and in FIG. 3 herein. ERR-CALC now incorporates the teachings of this invention. The Calculational Engine continuously monitors system heat rate on-line, i.e., in essentially real-time, as long as the thermal system is burning fuel. The application of this invention to The Input/Loss Method as taught in ""711 and installed as part of the Calculational Engine significantly enhances the power plant engineer""s ability to predict tube failures and reduce outage time required for repair.
The present invention provides a procedure for determining tube leaks in a fossil-fired thermal system such as a power plant or steam generator, using combustion stoichiometrics in combination with an ability to correct effluent data such that consistent fuel chemistry is computed.
The present invention teaches the mechanism of how a tube failure has been detected stoichiometrically, such detection being important to the system operator. Also, the present invention teaches how the location of a failed tube may be determined.
Other objects and advantages of the present invention will become apparent when its general methods are considered in conjunction with the accompanying drawings and the related inventions of ""932, ""711, ""853 and ""879.
This invention has been reduced to practice and installed for demonstration at two power plants to determine the operability and functionality of this invention. These demonstrations have produced outstanding results demonstrating several identified tube failures.